Standard circuit breakers of the type found in residential, institutional and light commercial settings are thermal/magnetic in operation. Requirement 250.4(A)(5) of the National Electrical Code (NEC) and the expectation of the electrical industry is that a circuit breaker should respond as rapidly as possible to a short-circuit or a ground-fault. The magnetic portion of the breaker is designed to react within 1 cycle on 60 Hz systems to the higher levels of overcurrent produced by parallel fault conditions such as short-circuits or ground-faults. Branch circuits, especially 15 and 20 A/120-volt circuits supplying general-purpose outlets in homes and buildings throughout North America and elsewhere, exhibit impedance characteristics that often limit the current available in a fault condition to relatively low levels by comparison to circuits of greater ampacity and/or voltage. This limitation results in available fault current at many outlets on these circuits that is insufficient to trigger the magnetic (1-cycle) response mechanism in the circuit breakers installed to protect them, leaving only the long-time (thermal) pick-up response of the circuit breaker as protection for parallel short-circuit or ground-fault overcurrent. This bi-metal thermal element is designed for inverse-time response to low-level overcurrents and may take up to several seconds to respond to a short-circuit or ground-fault. Because short-circuits or ground-faults have been shown in research to cause rapid ignition, the potential for an unsafe condition can result at an outlet at which the above condition exists.
At the present time the product listing requirement for standard circuit breakers (Underwriters' Laboratories, Inc. standard #489, Section 7) does not specify what level of overcurrent is required to engage the magnetic response of a thermal/magnetic circuit breaker that is intended to rapidly clear a short-circuit or ground-fault. Yet existing NEC requirement 250.4(A)(5) requires a circuit breaker to open as rapidly as possible in response to fault current. However, the product standard does not yet require an instantaneous response, and there is no test device that is capable of verifying in a practical and convenient manner that an installed equipment-grounding conductor and the circuit breaker of any branch circuit meet this requirement of the NEC, or that a given installed outlet will receive a rapid breaker response to a short-circuit or ground-fault. The lack of standardization of this particular breaker response in combination with a wide variation of available short-circuit and fault current at 120-volt outlets has resulted in a situation where there is a great disparity in short-circuit and ground-fault protection provided from premises to premises and even outlet to outlet in both the installed building infrastructure and in new buildings as they are wired today. The fact that this condition has perpetuated itself is in part a consequence of the lack of the necessary tools in the electrical service industry to verify compliance with 250.4(A)(5) of the NEC. As tools are developed that increase awareness of this condition, so will recognition of the need to test the instantaneous response of a circuit breaker.
U.S. Pat. Nos. 5,736,861 and 6,815,955 describe a method for measuring the impedance characteristics of branch circuits, and using the measurements to qualify the suitability of a circuit to safely sustain a short-circuit, and to calculate the maximum duration of a short-circuit of a given calculated magnitude that is inversely related to the measured branch impedance.